Fluid driven turbines have been used as in-hole motors positioned adjacent the bit in a drill string to rotate the bit by means of the drilling fluid pumped down the drill string from the surface through the turbine and the bit to be circulated up the annulus between the drill string and the bore hole.
In such motors the torque developed by the turbine to effect drilling is at given drilling fluid circulation rate inversely proportional to the speed of rotation (rpm - revolutions per minute) of the motor. Further, both the efficiency and the horse power developed by the motor passes through a maximum as the rpm increases, falling sharply as the rpm increases beyond the optimum rpm.
It is highly desirable for the driller to have information as to the rpm of the motor during drilling.
It has been suggested in the prior art (see Jeter, U.S. Pat. No. 3,065,416) to measure the rpm of an in-hole turbine motor connected to a bit, by generating variations in pressure in the drilling fluid pressure to the turbine, which is measured at the surface, responsive to the rpm of the turbine.
In such a device, the mechanism for moving the throttling valve is exposed to the drilling fluid which, in many cases, is drilling mud. The mud is a suspension of a thixotropic clay and in the usual case contains abrasive particles, which is that portion of the mud recirculated from the bore hole, which usual screening has not removed.